Collisions between rare-gas atoms at low keV energies. I. Symmetric systems

Elastic and inelastic collisions in the symmetric He-He, Ne-Ne, Ar-Ar, and Kr-Kr systems are investigated experimentally and theoretically at low keV energies (0.5-4 keV). Standard techniques, employing electrostatic energy analysis for ions and time-of-flight methods for neutrals, are used in energy-loss measurements. The characteristic energy spectra of the scattered beams generally show three peaks corresponding to elastic scattering and collisions resulting in one- or two-electron excitations. Of particular significance is the dominance of the lowest-lying $p$ levels in the inelastic scattering. Differential cross sections are determined at select energies and the interpretation of the results is made in the framework of the electron-promotion model. Ab initio calculations are performed for the Ne and Ar cases. The primary mechanism in the inelastic scattering always involves one- or two-electron transitions from a promoted incident molecular orbital by rotational coupling for He-He, and by radial coupling in the Ne-Ne and Ar-Ar cases. Predictions based on the assumed model are compared with results of related experiments.